refactor(agent): move the button runtime's worker loop into button/worker.rs

runtime/button.rs was 1022 lines: the press, session and event types, the input handle and the owner, and below them the 320 lines the worker thread runs — draining commands and inputs, settling long presses, and emitting events. The worker is button/worker.rs now; the owner spawns run_worker as before, and the four worker steps the tests drive directly are imported back under cfg(test).

Pure move: no line inside the moved functions changed beyond pub(super) on what the owner and the tests call. button.rs is 702 lines.